Process to prepare pyrimidine nucleosides

ABSTRACT

A process for the production of a compound of the formula: ##STR1## wherein R 1  is hydrogen, alkyl C 1  -C 16  substituted alkyl, aryl, substituted aryl, aralkyl, substituted aralkyls, alkenyl, substituted alkenyl, alkynyl, substituted alkynyl, cyano, carboxy, carboxy esters, carboxamido, N-mono substituted and N,N-disubstituted carboxamido with alkyl, aralkyl and aryl groups; R 2  is hydrogen, alkyl C 1  -C 16  substituted alkyl, aryl, substituted aryl, aralkyl, substituted aralkyls, alkenyl, substituted alkenyl, alkynyl, substituted alkynyl; R 3  is alkyl C 2  -C 6 , branched alkyl, aryl C 2  -C 6 , substituted aryl and R 4  is halogen or H. 
     A condensation reaction is effected between compounds of the formulae: ##STR2## to form a novel compound of the formula: ##STR3## Compound (2) is acylated with a compound of the formula: 
     
         R.sub.3 COCl 
    
      to form a novel compound of the formula: ##STR4## Compound (3) is mixed with pyridine and is reacted with thionyl chloride to form compound (4) and compound (4) is dehalogenated to form compound (5).

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application is a divisional of application Ser. No. 08/804,636filed Feb. 25, 1997, now U.S. Pat. No. 5,760,208, which is acontinuation-in-part of Ser. No. 08/696,535 filed Aug. 14, 1996, nowabandoned.

BACKGROUND AND BRIEF SUMMARY OF THE INVENTION

Pyrimidine nucleosides are important antiviral agents, increasedattention has recently been focused on these compounds with the FDAapproval of 3'-azido-2',3'-dideoxythymidine (AZT) stavudine (D₄ T) as aneffective treatment for Acquired Immunodeficiency Syndrome (AIDS). Sincethe synthesis of such agents utilizes the pyrimidine nucleosideβ-thymidine as a starting material, new methods for the low-costproduction of this and other synthetic intermediates are also becomingimportant. The present invention involves an expeditious route to theO²,2'-anhyro-1-(β-arabinofuranosyl) 5-hydroxy 5,6 dihydro pyrimidinenucleosides, a class of compounds easily converted to the β-pyrimidinederivatives. Analogous syntheses of these anhydronucleosides isdescribed in the following publications.

Japanese Kokai No. 81 49 398 laid open on May 2, 1981 refers to thesynthesis of acylated arabinofuranosylcyclothymine compounds. Theprocess of the Japanese Kokai requires that the iminoarabino(1 2:4.5)oxazoline acid addition salt be acylated.

In an article appearing in J. Mo. Biol., 1970, 47, 537 the authorsdescribe the use of a readily available amino-oxazoline carbohydratederivative as a useful precursor to a variety of anhydronucleosides.

In the reference Kampe, K. D.; Justus Leibigs AnnChem., 1974, (4),593-607 (ger), reactions of aminooxazolines with unsaturated esters aredisclosed. European patent application 0 351 126 discloses a process forthe formation of O²,2'-anhydro-1-(β-D-arabinofuranosyl)thymine byreacting 2-amino-β-D-arabinofuranooxazoline(s) with an alkyl 3-halo oralkoxy-methacrylate derivative.

In U.S. Pat. No. 5,077,403 discloses a process which starts withacrylates or acrylonitriles and related derivatives; compounds that areat a lower oxidation state than other substrates previously used forsimilar condensations This results in the formation of 5,6-dihydropyrimidine nucleoside derivatives which are then oxidized to therequired nucleosides in high to excellent yields.

Since the issuance of the '403 patent, the following articles have beenpublished.

Pragnacharyulu, Palle V. P.; Vargeese, Chandra; McGregor, Michael;Abushanab, Elie, Diastereomeric 5,-6-Dihydrothymidines. Preparation,Stereochemical Assignments, and MnO₂ Oxidation Studies to Thymidines,Journal of Organic Chemistry, pp. 3096-3099, 60, 1995;

Rao, A. V. Rama; Gurjar, Mukund K.; Lalitha, Sista V. S., Discovery of aNovel Route to β-Thymidine: a Precursor for anti-AIDS Compounds, J.Chem. Soc., Chem. Commun., pp. 1255-1256, 1994;

Barvian, Mark R.; Greenberg, Marc M., Diastereoselective Synthesis ofHydroxylated Dihydrothymidines Resulting from Oxidative Street, J. Org.Chem., 58, 6151-6154, 1993;

Sawai, Hiroaki; Nakamura, Akiko; Sekiguchi, Sumie; Yomoto, Keisuke;Endoh, Masakazu; Ozaki, Hiroaki; Efficient Synthesis of New5-Substituted Uracil Nucleosides Useful for Linker Arm Incorporation, J.Chem. Soc., Chem. Commun., pp. 1997-1998, 1994; and

Sawai, Hiroaki; Hayashi, Hidekazu; Sekiguchi, Sumie, Facile Synthesis of5-Substituted Arabinofuranosyluracil Derivatives, Chemistry Letters, pp.605-606, 1994.

Broadly, the present invention is directed to a process for theproduction of pyrimidine nucleoside compounds of the formula: ##STR5##and the enantiomers thereof, wherein R₁ is hydrogen, alkyl C₁ -C₁₆substituted alkyl, aryl, substituted aryl, aralkyl, substitutedaralkyls, alkenyl, substituted alkenyl, alkynyl, substituted alkynyl,cyano, carboxy, carboxy esters, carboxamido, N-mono substituted andN,N-disubstituted carboxamido with alkyl, aralkyl and aryl groups; R₂ ishydrogen, alkyl C₁ -C₁₆ substituted alkyl, aryl, substituted aryl,aralkyl, substituted aralkyls, alkenyl, substituted alkenyl, alkynyl,substituted alkynyl; R₃ is alkyl C₂ -C₆, branched alkyl, aryl C₂ -C₆,substituted aryl and R₄ is halogen or H.

A Michael type condensation reaction is effected on a compound of theformula: ##STR6## with a compound of the formula: ##STR7## to form anovel compound of the formula: ##STR8##

Compound (2) is acylated with a compound of the formula:

    R.sub.3 COCl

to form novel compounds (3) and (6) depending upon the duration of thereaction. ##STR9##

In the preferred embodiment, compound (3) is mixed with pyridine and isreacted with thionyl chloride to form compound (4) where R₄ =Cl.##STR10## Compound (4) is dihalogenated to form compound (5). ##STR11##

Compound (5) can be hydrolyzed by any well known process to formthymidine.

In an alternative embodiment of the invention, compound (6) is mixedwith pyridine and is reacted with alkyl or aryl sulfonyl halide to forma novel compound of the formula (7).

Compound (7) is then mixed with pyridine and thionyl chloride to form acompound of the formula (8). ##STR12## where R₆ is alkyl C₂ -C₆,branched alkyl, aryl C₂ -C₆, substituted aryl

Compound (8) can be converted to either AZT or d₄ t as described in thechemical literature, B. C. Chen. et al., Tetrahedron letters 36,7957-7960 (1995); B. C. Chen. et al., Tetrahedron letters 36, 7961-7964(1995).

The present disclosure embodies several inventive embodiments. Oneembodiment of the invention is a process for the production of compounds(4) and (5); another embodiment of the invention comprises the compound(2) and the method for the production of compound (2); anotherembodiment comprises compound (3) and the method for the production ofcompound (3) and the conversion of compound (3) into 5 hydroxy 5,6dihydro pyrimidines, e.g. AZT, d₄ t; another embodiment comprises novelcompounds (6) and (7) and the processes for the production of compounds(6) and (7); and lastly, another embodiment of the invention comprises aprocess for the production of compound (8).

BRIEF DESCRIPTION OF THE DRAWINGS

The FIGURE illustrates reaction schemes of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

Presently, there are several methods to produce prymidine nucleosides,however these methods are limited in their utility either because theycannot form 5 substituted prymidine derivatives directly or involve longsynthetic routes resulting in increased cost and reduced yields. Thesynthetic route disclosed in the '403 patent, although advantageous inview of the then prior art synthesis, still resulted in high costprimarily because of the oxidizing agent found most efficacious in theprocess, namely DDQ.

In the present invention the condensation reaction between the startingoxazoline (1) is effected with a compound which is itself oxidizedresulting in a new composition of matter, namely compound (2). Oneaspect of the invention embodies compound (2) and the method of makingcompound (6). Compound (2) is further reacted with pivaloyl chloride toform a new composition of matter, namely compound (3). Another aspect ofthe invention embodies compound (3) and the method of making compound(3). With the process of the invention the desired pyrimidine isproduced efficiently and in high yields.

The first step of the process of the present invention comprises thecondensation of an oxazoline derivative of formula (1) with a glycidateester or glycidonitrile of formula (1a) to yield derivatives of formula(2). This process is preferably carried out at a temperature of about50° C. to about 150° C., preferably at 80° C. to 100° C., in thepresence of reaction-inert solvent. Preferred solvents are dimethylacetamide, dimethyl formamide and other organic solvents such as C₁ -C₄alkanols, preferably methanol, dimethyl sulfoxide, acetone etc. can alsobe used. Water may also be used. Although preferred embodiment employsequimolar amounts of compounds (1) and glycidate ester (1a), an excessof either reagent may be used.

In the reaction between compounds (1) and the glycidate ester (1a), thepressure is not critical. Generally the reaction is conducted at apressure of from about 0.5 to about 2.0 atmospheres, preferably atambient pressure, (i.e. about one atmosphere).

The second step of the process comprises the selective acylation of (2)with concomitant opening of the anhydro bond by the halide ion at the2'-position. While prolonged reaction times (24 h) lead to thedi-acylated derivative (3), shortened reaction times (3 h) give thenovel mono-acylated derivative (6). The reaction is preferably carriedout in acetonitrile from about 50° C. to about 150° C., preferably at80° C. to 100° C. Other solvents such as THF, dimethyl formamide,dimethyl acetamide can also be used.

The final step of the process involves dehydration of compounds of theformula (3) and (7) to the corresponding pyrimidine nucleosides. Thus,(3) and (7) upon treatment with thionyl chloride in pyridine lead tocompounds of the formula (4) and (8) respectively. Other dehydratingagents such as trifluoroacetic anhydride, phosphorous pentoxide etc. canalso be used. Preferred temperatures for this reaction is -40° C. toabout 50° C., preferably 0-5° C. Other solvents such as trialkyl aminescan also be used.

Three major advantages of this process over those of the prior art arelow cost of the starting materials and the efficiency of the process, noα or β mixtures in the final product, which permits a high yield of thedesired compound and formation of new composition of matter (2) whichprovides access to novel and new 5-hydroxylated analogs of drugs inclinical use such as AZT and d₄ T.

CHEMISTRY Experimental2,2'-Anhydro-1-β-D-arabino-furanosyl-5-hydroxy-5,6-dihydrothymine (2)

A mixture of aminooxazoline derivative 1 (87 mg, 0.5 mmol) and methyl2-methyl glycidate (0.23 mg, 2 mmol) in toluene (5 mL) was heated at 90°C. for 48 h. The solvent was removed under reduced pressure and theresidue was chromatographed on silica gel eluting first with EtoAc toremove non-polar impurities followed by 15% EtoAc to give 2 as 1:1diastereomeric mixture (100 mg, 78%): ¹ HNMR (300 MHz, D₂ O) δ1.29 (s,3H) , 3.48-3.68 (m, 4H), 4.25-4.29 (m, 1H), 4.53-4.55 (m, 1H) , 5.30 (d,J=5.7 Hz, 1H), 6.01 (d, J=5.7 Hz), 6.06 (d, J=5.7 Hz), (1H).

Anal. Calcd for C₁₀ H₁₄ N₂ O₆ : C, 46.51; H, 5.46; N, 10.84. Found: C,46.67; H, 5.87; N, 10.63.

1-(3',5'-Di-O-Pivaloyl-2'-chloro-2'-deoxy-β-D-ribofuranosyl)-5-hydroxy-5,6-dihydro-thymine(3)

Compound 2 (200 mg, 0.75 mmol) and pivaloyl chloride (1.5 mL) weredissolved in AcCN (15 mL) and the mixture was refluxed for 24 h. AcCNwas removed under reduced pressure and the residue was partitionedbetween CH₂ Cl₂ and aq. NaHCO₃, water and brine, respectively. Removalof CH₂ Cl₂ gave, after silica gel column chromatography (20%<<50%EtoAc/hexanes), the title compound 3 as diastereomeric mixture (190 mg,57%): ¹ H NMR (300 MHz, CDCl₃) δ1.24 (s, 9H), 1.26 (s, 9H), 1.52 (s)1.66 (s) (3H), 3.36 (AB quartet centre, Δu_(AB) =61.2 Hz, J_(AB) =12.8Hz, 2H), 4.16-4.49 (m, 5H, 1 H D₂ O exchangeable), 5.24 (dd, J=5.9, 2.7Hz, 1H), 6.09 (d, J=7.3 Hz), 6.1 (d, J=8.1 Hz) (1H), 8.68 (s) 8.84 (s)(1H, D₂ O exchangeable).

1-(3',5'-di-O-Pivaloyl-2'-chloro-2'-chloro-2'-deoxy-β-D-ribofuranosyl)thymine(4)

Compound 3 (30 mg, 0.069 mmol) was taken in pyridine (2 mL) and themixture was cooled in an ice bath under argon. Thionyl chloride (0.1 mL,1.36 mmol) was added dropwise and the mixture was stirred at the sametemperature for 3 h. CH₂ Cl₂ (30 mL) was added to the reaction mixtureand the organic layer was washed with dil. HCl, water and brine. Removalof CH₂ Cl₂ gave a residue which was purified by column chromatography(20% EtOAc/hexanes) to give 4 (16 mg, 60%). ¹ H NMR (90 MHz, CDCl₃) 1.3(s, 18H), 1.95 (s, 3H), 4.0-4.6 (m, 4H), 5.1-5.3 (m, 1H), 6.1 (d, J=7.0HZ, 1H), 7.25 (S, 1H), 8.9 (bs, 1H, D₂ O exchangeable).

1-(3',5'-Di-O-pivaloyl-2'-deoxy-β-D-ribofuranosyl)-thymine (5)

Tributyltin hydride (100 mg, 0.36 mmol), azabisisobutyronitrile (5 mg)and compound 5 (43 mg, 0.1 mmol) were dissolved in anhydrous toluene (5mL) and refluxed for 3 h. The residue obtained after concentrating thereaction mixture was purified by column chromatography over silica gelusing EtOAc/hexanes (1:1) as eluent to provide 5 (35 mg, 85%) as a gum.¹ H NMR (90 MHz, CDCl₃) 1.3 (s, 18H), 1.7-2.6 (m, 5H), 3.9-4.5 (m, 3H),5.0-5.2 (m, 1H), 6.1-6.3 (dt, 1H), 7.3 (s, 1H), 9.3 (bs, 1H).

1-(5'-O-Pivaloyl-2'-chloro-2'-deoxy-β-D-ribofuranosyl)-5-hydroxy-5,6-dihydro-thymine(6)

Compound 2 (258 mg, 1 mmol) and pivaloyl chloride (1.0 mL) weredissolved in AcCN (15 mL) and the mixture was refluxed for 24 h. AcCNwas removed under reduced pressure and the residue was partitionedbetween CH₂ Cl₂ and aq. NaHCO₃, water and brine, respectively. Removalof CH₂ Cl₂ gave, after silica gel column chromatography (20%<<50%EtoAc/hexanes), the title compound 6 as diastereomeric mixture (200 mg,53%): ¹ H NMR (90 MHz, CDCl₃) 1.2 (s, 9H), 1.9 (s, 3H), 3.0 (bs, 2 H, D₂O exchangeable), 3.4 (AB quartet centre, Δu_(AB) =21 Hz, J_(AB) =23 Hz,2H), 4.0-4.5 (m, 5H) 6.0-6.2 (m, 1H), 8.8 (bs, 1H, D₂ O exchangeable).

1-(5'-O-Pivaloyl-3'-p-toluenesulphonyl-2'-chloro-2'-deoxy-β-D-ribofuranosyl)-5-hydroxy-5,6-dihydro-thymine(7)

Compound 6 (40 mg, 0.1 mmol) was taken in pyridine (2 mL) and themixture was cooled in an ice bath under argon. P-toluenesulphonylchloride (100 mg) was added and the mixture was stirred at the roomtemperature for 3 h. CH₂ Cl₂ (30 mL) was added to the reaction mixtureand the organic layer was washed with dil. HCl, water and brine. Removalof CH₂ Cl₂ gave a residue which was purified by column chromatography(20% EtOAc/hexanes) to give 7 (40 mg, 71%). ¹ H NMR (90 MHz, CDCl₃) 1.3(s, 9H), 1.5 (s, 3H), 2.8 (bs, 1H, D₂ O exchangeable), 3.1-3.4 (m, 2H),4.1-4.5 (m, 4H), 4.9-5.1 (m, 1H), 5.9 d, J=7.0 Hz, 1H) 7.4 (d, J=7.0 Hz,2H), 7.9 (d, J=7.0 Hz, 2H), 8.4 (bs, 1H, D₂ O exchangeable).

The preferred embodiment of the invention has been described withreference to a starting compound of D-arabinose to synthesizeβ-D-nucleosides and particularly 2' deoxy nucleosides. As would beunderstood by one skilled in the art, the enantiomer of D-arabinose,namely L-arabinose can be used as a starting compound. The L-arabinosecan be used with the same reactants and/or reagents and under the samereaction conditions, described for the preferred embodiment, to produceα-L-nucleosides. The reaction scheme to produce α-L-nucleosides is setforth below. ##STR13##

The foregoing description has been limited to a specific embodiment ofthe invention. It will be apparent, however, that variations andmodifications can be made to the invention, with the attainment of someor all of the advantages of the invention. Therefore, it is the objectof the appended claims to cover all such variations and modifications ascome within the true spirit and scope of the invention.

Having described our invention, what we now claim is:
 1. A process forthe production of a compound of the formula: ##STR14## and enantiomersthereof where R₁ is hydrogen, alkyl, C₁ -C₁₆ substituted alkyl, aryl,substituted aryl, aralkyl, substituted aralkyls, alkenyl, substitutedalkenyl, alkynyl, substituted alkynyl, cyano, carboxy, carboxy esters,carboxamido or N-mono substituted and N,N-disubstituted carboxamidosubstituted with alkyl, aralkyl or aryl groups;where R₂ is hydrogen,alkyl, C₁ -C₁₆ substituted alkyl, aryl, substituted aryl, aralkyl,substituted aralkyls, alkenyl, substituted alkenyl, alkynyl orsubstituted alkynyl; which comprises: effecting a condensation reactionon a compound of the formula: ##STR15## with a compound of the formula:##STR16##
 2. A composition comprising both enantiomers about positionC-3 in the pyrimidine ring of structure (2) wherein R₁ is hydrogen,alkyl, C₁ -C₁₆ substituted alkyl, aryl, substituted aryl, aralkyl,substituted aralkyls, alkenyl, substituted alkenyl, alkynyl, substitutedalkynyl, cyano, carboxy, carboxy esters, carboxamido or N-monosubstituted and N,N-disubstituted carboxamido substituted with alkyl,aralkyl or aryl groups; andwhere R₂ is hydrogen, alkyl, C₁ -C₁₆substituted alkyl, aryl, substituted aryl, aralkyl, substitutedaralkyls, alkenyl, substituted alkenyl, alkynyl or substituted alkynyl.